Hi everyone! First of all, i don't know if this idea is already being discussed so if there's a discussion on this i'd like to follow. To illustrate, first i want to create a class like: class Has a t where get :: t -> a then i define instance for simple product type such as tuple: instance Has a (a, b) where get (a, _) = a instance Has b (a, b) where get (_, b) = b ... You can image i will use th to make lots of instance for difference tuple size. Now if i want an extensible reader, i use Has class like this: someReader :: Has Int t => Reader t Int someReader = do x <- ask return $ get x + 1 Then i can run it with any tuple with an Int field like: runReader someReader (0 :: Int, "adad”) -- 1 This typeclass almost solved all problem of my network application: sometime’s i want ensure a logger, a sql backend and a http client pool in my monad’s environment, but i don’t want to fix my environment into a record. We can add a set :: a -> t -> t, or use lens to define Has, so that we can have extensible states. We can also use Tagged to achieve something like: (Has (Tagged “SqlBackEndOne” SqlBackEnd) t, Has (Tagged “SqlBackEndTwo" SqlBackEnd) t) => Reader t () It there a library doing this, maybe in lens? or there’re some drawbacks i didn’t notice? All ideas are welcomed! Cheers~ Winter